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The basic component materials of grinding wheels are abrasive mineral grains and bonding material. The abrasive mineral or synthetic mineral grains must be selected with respect to material that need to be cut or finished. The abrasive grains can be made from diamond, silicon carbide or aluminum oxide. The bonding material in which the abrasive grains are fixed is usually made either from organic materials (rubber, resin), or inorganic materials (clay). The inorganic materials allows medium to fine sizes of grains, while the organic material allows large sizes of grains.
The stages of manufacturing of the grinding wheel usually are the followings:
- step 1: the abrasive grains and the bonding material are mixed together;
- step 2: the mixture is poured into a 4 pieces mold and then is compressed by a hydraulic press;
- step 3: the wheel is fired at a temperature that depends on the bonding material: up to 200^o Celsius for organics and up to 1260^o Celsius for inorganic materials;
- step 4: the circumference of wheel is made concentric to its center in order to correct relative position tolerances.
The grinding manufacturing process is an alternative to the chip-removing manufacturing process and it is successfully used when very-hard materials are machined.
Hence, the grinding wheels tools are used either to cut very hard materials, or to finish the surface of parts. The grinding wheels recommended for cutting steel are the ones whose bonding material is resin, while grinding wheels recommended for finishing surfaces are the ones whose bonding materials are vitrified bonds.
The manufacturing process for grinding wheels begins with the selection of raw materials. The next step is mixing the raw materials and calibration of the proportions of each needed. This is followed by molding the mixture in a four-part mold form. After this, pressure force is applied to the mixture as it rests in the mold, forcing the materials together. Next, the discs of the grinding wheel are removed from the mold and, after final shaping, are fired in a kiln. The firing process melts the binding material and changes it to a resistant form.
Selecting Raw Materials
The two major components of all grinding wheels are the (1) abrasive mineral grains or synthetic mineral grains and the (2) bonding material. The earliest grinding material is thought to be sandstone containing (1) abrasive quartz mineral crystals held together in (2) natural earthen cement (e.g., clay minerals, calcite, silica or iron oxides). In the 1800s, emery was imported from India to use in Europe, England and the United States as grinding wheel abrasive grains. The cost of emery importation led to a search for less costly abrasives and, before emery was found to be plentiful in the United States, the search led to the development of chemically produced synthetic silicon carbide and synthetic corundum, an aluminum oxide, which is a bauxite derivative (bauxite: impure mixture of earthy hydrous aluminum oxides and hydroxides).
corundum: a very hard mineral that consists of aluminum oxide occurring in massive and crystalline forms, that can be synthesized, and that is used for gemstones (as ruby and sapphire) and as an abrasive (Merriam Webster Dictionary)
These synthetics proved more reliable and effective than abrasives in natural minerals. This success in the synthesis process spurred on further research that eventually led to the discovery of superabrasives, such as synthetic diamonds, synthetic cubic boron nitride and the newer seeded-gel aluminum oxide having a nano micro-structure built from sub-micron crystals. While these abrasives make grinding wheels effective, of equal importance is the bonding material that holds the grains together: without effective bonding the friability is undependable making the wheel unreliable or even useless (friability: the degree to which material is easily crumbled or reduced to powder, made crumbly as in friable rock). When rubber and clay were introduced as components of abrasive bonds in 1840, the success of grinding wheels was firmly established because the friability was reduced and controlled. In the 1870s another step forward in bond material occurred when vitrified bond structure was patented (vitrified: converted by heat into glass). Vitrified bonds continue to berefined yet grinding wheels composed of vitrified materials contained in a bonding matrix continue to be of great importance even though other processes of manufacturing grinding wheels are available, for example, "bonding a layer of abrasives to the surface of a metal wheel" (Theodore L. Giese, "Grinding Wheel" on Gale Cengage).
vitrified bond: A clay or ceramic bond characterized by its strength, rigidity, and resistance to oils, water, or temperature changes. (ToolingU.com)
Mixing Raw Materials
Abrasive grains and bond are the primary components mixed together in the grinding wheel manufacturing process but, depending on the grinding needs, other additives, such as sawdust, crushed nut shells, and coke (coke: a hard, porous derivative of coal or made synthetically from petroleum products) may be added to "create proper porosity and spacing" (Giese). Other materials, like napthaline-wax, that vitrify during the kiln firing process are currently preferred over materials, like coke, that do not vitrify. Another class of additives are called aides to grinding, like sulfur and chlorine compounds, and enhance metal-cutting properties by inhibiting reactive microscopic welding of ground metal particles in response to the grinding action. The raw materials and additives are mixed together according to a specified formula with great care and consideration given to friability, vitrification and to creating a wheel with the necessary properties for shaping the a specific material in the desired manner (Giese).
In the mixing stage, the abrasive is mixed with the binder, which will become the bond after vitrifying kiln firing is completed. The binder coats the abrasive grains so that they adhere to the binder, which is also instrumental in the wheel retaining its shape "until the bond is solidified." This mixture is then called the "blend" and the blend is free-flowing with the abrasive crystals distributed evenly throughout, a point critical to creating an effective grinding wheel so as to "assure uniform cutting action and minimal vibration" during use (Giese).
The blend is, or grinding wheel mixture, is poured in carefully predetermined amounts into four-part mold shapes: (1) a central pin provides the wheel's center hole, called an "arbor hole; (2) a external shell to contain the free-flowing mixture that has a thick 1-inch wall and is twice the height of the desired wheel thickness; (3, 4) two flat disks of the same dimensions of the finished wheel dimensions so the center hole and edges match what will be the finished wheel. To ensure even distribution of blend in the mold, a flat edge arm pivots around the center hole thus spreading and leveling the blend as it is poured. While in the mold, pressure "in the range of 100 to 5000 pounds per square inch (psi)" is applied for between 10 to 30 seconds to the two flat disks covering the top and the bottom of the mold. This pressure compacts the blend into it's final size and shape (Giese). The mold is then removed from the pressure force press and the wheel is carefully stripped from the mold, carefully adjust to conform to its final shape, then transported on heat resistant carriers to the kiln where the firing and vitrification process takes place.
The firing process heats the binder to the final bond that will solidify the final grinding wheel and will suspend the abrasive crystal grains in a useable wheel. A fired wheel will be resistant to heat during high-speed grinding and resistant to cleaning solvents. Depending upon the types of wheels being manufactured, a wide range of furnaces, kilns and temperatures are used. For example, "wheels with vitrified bonds are fired to temperatures between 1700 and 2300 degrees Fahrenheit (927 to 1260 degrees Celsius)" (Giese).
Final product quality adjustments are made such as reaming the center so it is concentric with the edges, adjusting for circumference shape and wheel thickness and to add special contours to the wheel. Large wheels may be balanced to reduce vibration generated during use. Manufacturer labels and markings are applied, then the wheels are ready for shipment.
[C]ompetition from several alternative technologies is likely to grow. Advances in cutting tools made of polycrystalline superabrasive materialsine grain crystalline materials made of diamond or cubic boron nitrideill make such tools a viable option for shaping hard materials. Also, advances in the chemical vapor deposition of diamond films will affect the need for abrasives by lengthening the life of cutting tools and extending their capabilities. (Giese)
Source: Theodore L. Giese, "Grinding Wheel." How Products are Made. Ed. Stacey L. Blachford. Vol. 1. Gale Cengage, 2002.
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